In recent years, there is a global tendency toward stricter quality regulation values for diesel oil so as to improve the atmospheric environment. Particularly, since there is a concern that a sulfur content in diesel oil may adversely influence the durability of after-treatment apparatuses expected as countermeasures against exhaust gas, such as oxidation catalysts, nitrogen oxide (NOx) reduction catalysts, and continuous regenerating type diesel exhaust particle removing filters, it is required to reduce the sulfur content in diesel oil.
Under the above circumstances, it has been emphasized to develop an ultra-deep desulfurization technology for substantially removing most of the sulfur content in diesel oil. The general technology for reducing the sulfur content in diesel oil is to use severer operating conditions for hydrodesulfurization, for example, reaction temperature and liquid hourly space velocity. However, when the reaction temperature is raised, carbonaceous matter precipitates on the catalyst and the activity of the catalyst is rapidly lowered. In addition, when the liquid hourly space velocity is decreased, desulfurization ability is improved but a purification capacity is lowered. Thus, it is necessary to enlarge the scale of a facility.
Consequently, the best way of attaining the ultra-deep desulfurization of diesel oil without using severer operating conditions is to develop a catalyst having excellent desulfurization activity. In recent years, many investigations have been made on types of active metals, methods of active-metal impregnation, improvements of catalyst supports, regulation of catalyst pore structures, activation methods, and the like, and novel catalysts for ultra-deep desulfurization development have been reported. For example, PTL 1 discloses a hydrogenation desulfurization catalyst which supports active metals, phosphorus, and an organic acid on an inorganic oxide support containing alumina or obtained by incorporating zeolite, boria, silica, zirconia or the like into alumina.